System for vaporization of liquid fuels for combustion and method of use

ABSTRACT

A gas stream with a reduced oxygen concentration relative to ambient air is used to vaporize a liquid fuel or liquefied higher hydrocarbon gas, or is mixed with a vaporized gas, and the reduced oxygen vaporized fuel gas is fed to a combustion device such as a premixed or diffusion combustor. Preferably, the oxygen content of the gas stream is less than the limiting oxygen index. By mixing the fuel with a gas stream that has an appropriately reduced oxygen content, auto-ignition prior to the desired flame location in the combustor can be avoided. In some embodiments, the reduced oxygen stream is generated from an air separator or taken from the exhaust of the combustion device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/929,675, filed Oct. 30, 20007, which is a continuation of U.S.application Ser. No. 11/464,441, filed Aug. 14, 2006, which is adivisional of U.S. application Ser. No. 10/682,408, filed Oct. 10, 2003,which claims priority to U.S. Provisional Application No. 60/417,184,filed Oct. 10, 2002; and U.S. Provisional Application No. 60/430,653,filed Dec. 4, 2002. The entireties of all of the aforementionedapplications are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to methods and devices for suitablyvaporizing, mixing, and delivering liquid fuels or liquefied gases foruse in combustion devices.

2. Background of the Technology

Combustion devices, such as gas turbines used for power generation, aretypically fueled by natural gas (e.g., compressed natural gas or CNG).Typically, natural gas consists of approximately 90-98% by volumemethane (CH₄), although some gases with as little as 82% methane havebeen characterized as natural gas. Other than methane, natural gas mayinclude CO₂, 0₂, N₂ and higher hydrocarbon gases, such as C2 (ethane,ethylene, acetylene), C3 (propane), C4 (butane), and C5 (pentane).

Recent advances in the design of the combustion systems for gas turbineengines have resulted in substantial improvements in exhaust emissionsduring operation on natural gas through the use of lean, premixedcombustion. In this combustion mode, natural gas is premixed withcombustion air prior to arrival at the flame front. This lean mixture ofnatural gas and air burns at a lower temperature than conventionaldiffusion flame combustors, thereby producing lower levels ofpollutants, including oxides of nitrogen (NO_(x)) in the exhaust stream.By way of example, the maximum allowable NO_(x) levels for diffusionflame combustors is typically 42 ppm @ 15% 0₂, while the maximumallowable NO_(x) levels for a lean, premixed combustion gas turbine isnow typically 15 ppm @ 15% 0₂. The 42 ppm NO_(x) level for diffusionflame combustors generally can only be achieved through the addition oflarge amounts of steam or water into the combustor to reduce the flametemperature.

Attempts have been made to operate lean, premixed combustion deviceswith alternate, higher hydrocarbon liquid fuels such as oil and dieselfuel and higher hydrocarbon fuel gases such as propane (C3), and butane(C4). As used herein, “higher hydrocarbon fuel” refers to a fuel whereinat least 50 weight percent of the hydrocarbon molecules of the fuel haveat least two carbon atoms. Unfortunately, these combustion devicescannot be readily operated in a lean, premixed, pre-vaporized (LPP)combustion mode when using the alternate liquid fuels. In order togenerate a lean, premixed, prevapor-ized flame using liquid fuels orliquefied gases (as used herein, the term “liquid fuel” should beunderstood to include fuels that are normally in a liquid state at roomtemperature and atmospheric pressure, as well as gases that have beenliquefied by cooling and/or pressurizing), the liquids must first beevaporated into a carrier gas (normally air) to create a fuel gas (i.e.a fuel vapor/air mixture) which then may be mixed with additionalcombustion air prior to arrival at the flame front. However, aphenomenon known as auto-ignition can occur with such vaporized liquidfuel/liquefied gas and air mixtures. Auto-ignition is the spontaneousignition of the fuel prior to the desired flame location in thecombustion device. This premature ignition can occur, for example, as aresult of normal, premature, or other heating of the fuel that can occuras the fuel is fed to the combustion device. Auto-ignition results indecreased efficiency and damage to the combustion device, shortening theuseful life of the combustion device and/or causing an increase inunwanted emissions.

Various attempts have been made to curtail auto-ignition of higherhydrocarbon liquid fuels in such lean, premixed combustion devices, butnone of them have proven entirely successful. As a result, “dual fuel”combustion devices, such as gas turbines, capable of operating with bothnatural gas and higher hydrocarbon liquid fuels typically operate in alean, premixed mode when used with natural gas and in a diffusion modewhen used with higher hydrocarbon liquid fuels. Combusting the liquidfuels in the diffusion mode is undesirable as it increases NO_(x) andother emissions as compared to natural gas combusted in the lean,premixed mode.

Another issue that has recently become of increased importance is aproblem associated with the use of liquefied natural gas. A recentshortage in the domestic natural gas supply has made the importation ofliquefied natural gas more common. When liquefied natural gas isshipped, typically via tanker, the higher hydrocarbon gases have ahigher boiling point. When the liquid natural gas is re-vaporized foruse as a gaseous fuel, the last portion of liquefied natural gas removedfrom the storage container contains a higher percentage of higherhydrocarbon fuels. Due to the aforementioned auto-ignition problem, thisportion of the liquefied natural gas cannot be used in many existinglean, premixed natural gas combustors.

Combustion devices similar to those used with natural gas are also usedon boilers, incinerators, and turbine engines, and other combustionengines, including applications other than power generation, such as forpropulsion for naval ships. Problems with use of turbine engines fornaval ships include the large amount of storage space typically requiredfor conventional compressed gas fuel and high emissions that result fromalternative fuel use in conventional turbine engines. The emissions canboth violate environmental requirements and present a security hazardby, for example, producing visible emissions that reveal the position ofthe vessel.

There remains an unmet need for combustion devices such as turbineengines and other combustion devices that can be operated with bothnatural gas and higher hydrocarbon liquid fuels in a lean, premixed,pre-vaporized mode. A satisfactory dual fuel option for such combustiondevices would allow, for example, cost and fuel flexibility forapplications such as power generation and others.

SUMMARY

Embodiments of the present invention address the aforementioned issues,as well as others, to a great extent by providing a mechanism forproducing pre-vaporized fuel gas with a reduced oxygen content relativeto ambient air from a wide variety of liquid fuels or liquefied gases,which can be fed into a combustion device as a gaseous fuel. Inpreferred embodiments, the pre-vaporized fuel gas can be used withexisting lean, premixed combustion devices configured to combust naturalgas. Such a gaseous fuel feed is usable with turbine engines and dieseland gasoline engines, such as to power naval vessels, locomotives,aircraft, and automobiles. The invention is also usable with a widerange of other combustion devices, especially for combustion devices forwhich a high degree of ignition and/or emissions control is desired. Forexample, NO_(x) reductions can be achieved using the invention even withdiffusion flame combustors. This emissions reduction is achieved as aresult of the added heat capacity of the reduced oxygen stream/fuel gasmixture, since the additional inert gas serves to reduce flametemperature, thus reducing NO_(x).

In an embodiment of the present invention, an inert gas stream or othergas stream with a reduced oxygen concentration relative to air is usedto vaporize a liquid fuel or liquefied higher hydrocarbon natural gas,and the reduced oxygen vaporized fuel gas is fed to a combustion device.By mixing the fuel with a gas stream that has an appropriately reducedconcentration of oxygen, reaction of the vaporized fuel can be preventedor sufficiently delayed so as to avoid auto-ignition. A high degree ofignition control, as well as other features of the present invention, asdescribed further below, are usable to reduce or otherwise controlemissions or combustion instabilities.

A number of devices or systems known in the art may be used to supplythe inert gas stream, and a number of inert gases may be used inconjunction with the present invention. For example, in one embodimentof the present invention, vitiated exhaust gas from a pre-burner or fromdownstream of the combustion device can provide a reduced oxygen streamfor vaporization of the liquid fuel or liquefied gas for use that avoidsauto-ignition. By appropriately conditioning this exhaust gas stream,the stream can be used to vaporize any of a variety of liquid fuels orliquefied gases, which, once appropriately processed and mixed with theexhaust gas stream can be fed directly into a combustion device as agaseous fuel. In another embodiment of the present invention, an airseparator unit supplies the reduced oxygen gas stream to the liquid fuelor liquefied gas vaporizer.

Advantageously, this allows for a self-contained unit for producing apre-vaporized fuel from any of a variety of liquid fuels or liquefiedgases and compressed air, which, once appropriately processed and mixed,can be fed directly into an existing turbine engine adapted to combustnatural gas. This mixture can then be burned in a lean, premixed flamein order to improve engine performance. For example, such improvementsmay include, but are not limited to, improved exhaust emissions and/orgreater flame stability, including reduced combustion device dynamics.

An air separator unit for use in embodiments of the present inventionseparates oxygen and nitrogen from air. The output of the air separatorincludes two gas streams, a first stream that has increased oxygen andreduced nitrogen (“the oxygen-rich stream”) relative to air, and asecond stream that has reduced oxygen and increased nitrogen relative toair (the resulting reduced oxygen stream of this embodiment, as well asthe otherwise reduced oxygen streams of other embodiments, are referredto interchangeably as “the oxygen-reduced stream” or “the reduced oxygenstream”). In one embodiment of the present invention, the air separatorproduces the streams using a process referred to in the art as“adsorption.”

The oxygen-reduced stream may then be combined with vaporized liquidfuel or liquefied gas before being fed to the combustion device. Becausevaporized fuel requires a sufficient presence of oxygen in order tocombust, by mixing the vaporized fuel with an oxygen-reduced stream,such as an appropriate level of non-combustible nitrogen combined with areduced level of oxygen, combustion of the vaporized fuel can beprevented or sufficiently delayed so as to avoid auto-ignition. Thecombined fuel and oxygen-reduced stream may then be fed as a gaseousfuel into the combustion device, where the fuel/oxygen-reduced streammay be mixed with an oxygen source (e.g., intake air) for combustion inthe engine.

In an embodiment of the present invention, the air separator usescompressed air fed from the turbine compressor. Alternatively oradditionally, the air separator may use compressed air from anycompressed air source.

In one embodiment of the present invention, the oxygen-rich streamproduced by the air separator may be fed to the combustion devicedownstream of fuel burning in order to reduce emissions from the turbineengine. The feeding of an oxygen rich stream into the post-combustionemission stream can reduce the pollutants produced by the combustiondevice by, for example, enhancing the oxidation of unburned fuel and/orcarbon monoxide in the exhaust stream.

In one embodiment of the present invention, the oxygen-rich streamproduced by the air separator may be fed to the combustion device towiden the operating range of the combustion device.

Many liquid hydrocarbon fuels are usable with the present invention.Such liquid fuels or liquefied gases include but are not limited to,diesel fuel, #2 heating oil, gasoline, liquefied natural gas withelevated higher hydrocarbon content, other liquefied gases includingliquefied C2, C3, C4, C5, etc., and flammable liquid waste streams, suchas waste streams produced by manufacturing processes.

In one embodiment of the present invention, the heating value on a massor volumetric basis of the fuel gas stream may be controlled by mixingan appropriate proportion of the reduced-oxygen stream. This facilitatessupplying the fuel gas to the combustion device through, for example, anexisting natural gas fuel system.

Additional advantages and novel features of the invention will be setforth in part in the description that follows, and in part will becomemore apparent to those skilled in the art upon examination of thefollowing or upon learning by practice of the invention.

DESCRIPTION OF THE FIGURES

FIG. 1( a) is a block diagram of an embodiment of the present invention;

FIGS. 1( b) and 1(c) are block diagrams of different types of combustorssuitable for use in the embodiment of FIG. 1( a);

FIG. 2 shows a flow diagram of a method of using liquid fuels orliquefied gases and a combustion device, in accordance with anembodiment of the present invention;

FIG. 3 is a block diagram of an example gas turbine engine with a liquidfuel or liquefied gas combustion device for use therewith, in accordancewith an embodiment of the present invention;

FIG. 4 shows a flow diagram of a method of using liquid fuels orliquefied gases with a gas turbine engine, in accordance with anembodiment of the present invention;

FIG. 5( a) is a block diagram of an example gas turbine engine with aliquid fuel or liquefied gas combustion device for use therewith, inaccordance with an embodiment of the present invention;

FIGS. 5( b), (c), (d) and (e) are block diagrams of variousconfigurations of combustors of the gas turbine engine of FIG. 5( a);and

FIG. 6 shows a flow diagram of a method of using liquid fuels orliquefied gases with a gas turbine engine, in accordance with anembodiment of the present invention.

FIG. 7 shows a block diagram of a system employing a pre-burner tosupply a reduced oxygen gas stream in accordance with another embodimentof the invention.

FIG. 8 shows a block diagram of a system with a natural gas fuelmetering system in accordance with another embodiment of the invention.

DETAILED DESCRIPTION

The present invention will be discussed with reference to preferredembodiments of combustion systems. Specific details, such as types offuels and oxygen contents of gas streams, are set forth in order toprovide a thorough understanding of the present invention. The preferredembodiments discussed herein should not be understood to limit theinvention. Furthermore, for ease of understanding, certain method stepsare delineated as separate steps; however, these steps should not beconstrued as necessarily distinct nor order dependent in theirperformance.

As used herein, “vaporizing” should be understood to be distinct from“gasifying.” Gasifying is a term of art that refers to a process bywhich a non-gaseous fuel such as coal is converted into a gaseous fuelby partially reacting (e.g., burning) the non-gaseous fuel with ambientair or an oxygen-enriched gas stream. In contrast, reaction of theliquid fuel is substantially suppressed during the vaporizing processaccording to the present invention due to the presence of a gas streamwith reduced oxygen content relative to ambient air.

The invention is believed to be particularly applicable to lean,premixed, prevaporized combustion devices and therefore will bediscussed primarily in that context herein. However, the inventionshould not be understood to be so limited. For example, the inventionmay also be practiced with RQL (rich quenched lean) combustion devices,partially premixed combustion devices, or with diffusion flamecombustion devices.

Shown in FIG. 1( a) is a block diagram of a combustion system accordingto one embodiment of the invention including a typical combustor 5 (alsoreferred to herein interchangeably as a “combustion device”) for usewith liquid fuels or liquefied gases for a combustor, such as, but notlimited to, a turbine engine or a spark ignition or compression ignitionengine. As shown in FIG. 1( a), a liquid fuel/liquefied gas vaporizationunit I is connected to the combustor 5. A flow 8 of reduced oxygenvaporized fuel is provided to the combustor 5 from the vaporization unit1. Also input to the combustor 5 is an oxygenated gas stream 9, such asa source of air. In one embodiment, the combustor 5 includes featuresfor suitably mixing the vaporized fuel flow 8 and the flow of theoxygenated gas stream 9.

The vaporization unit 1 includes a reduced oxygen gas stream source 2, aliquid fuel/liquefied gas source 3 (also referred to hereininterchangeably as “liquid fuel” and/or “liquidized fuel”), and avaporizer unit 4. The liquid fuel/liquefied gas vaporization unit 4mixes and vaporizes the supply streams 6,7 from the liquidfuel/liquefied gas source 3 and the reduced oxygen gas stream source 2,respectively. Many different methods may be used to vaporize the liquidfuel stream 6 and the reduced oxygen gas stream 2. The order in whichthe mixing and vaporizing occurs is not important. In some embodiments,the mixing and the vaporization occur simultaneously, such as when thereduced oxygen stream is pre-heated to a temperature sufficient tovaporize the liquid fuel. In other embodiments, the liquid fuel stream 6is partially or completely vaporized, e.g., by heating the liquid fuel,prior to mixing with the reduced oxygen gas stream 7. In someembodiments, the reduced oxygen gas stream 7 is pressurized and/orheated prior to mixing and vaporizing The vaporized fuel stream 8, whichhas been conditioned to avoid auto-ignition by mixing with theoxygen-reduced stream, is then fed to the combustor 5 for use in thecombustion process.

In some embodiments, the vaporized fuel stream 8 is at a temperaturesufficiently high that the temperature of the vaporized fuel stream 8remains above the dew point during transit to the combustor 5. In otherembodiments, the temperature of the vaporized fuel stream 8 may fallbelow the de point if the distance that the vaporized fuel stream 8 musttravel to reach the combustor 5 is short enough such that there isinsufficient time for significant amounts of condensation to occur. Inyet other embodiments, the vaporized fuel stream 8 is heated between thevaporizer 4 and the combustor 5.

The reduced oxygen gas stream source 2 produces a gas stream with anoxygen content that is reduced relative to ambient air, which iscommonly taken as containing approximately 21% 0₂. In some embodimentsof the invention, the reduced oxygen gas stream has an oxygen contentbelow the limiting oxygen index. The limiting oxygen index (LOI) is theconcentration of oxygen in the local environment below which a materialwill not support combustion and varies for different types of liquidfuels. The LOI is typically between about 10% and about 14% and isapproximately 13% for many higher hydrocarbon fuels. The more the oxygencontent of the gas stream from the source 2 is reduced, the moreauto-ignition is suppressed. However, more work (i.e., energy) isrequired to produce a gas stream with a lower oxygen content. This workwill reduce the overall efficiency of the system. Thus, in someembodiments, the oxygen content from the stream source 2 is just lowenough to suppress auto-ignition by the required amount, which may beabove or below the LOI. In other embodiments of the invention, thereduced oxygen gas stream source 2 contains no oxygen. In some of theseembodiments, the gas supplied by reduced oxygen gas stream source 2 isinert; in yet other embodiments, the gas from source 2 containshydrocarbons (e.g., methane and/or higher hydrocarbons).

The amount of reduction in oxygen content in the gas stream from thesource 2 necessary to sufficiently suppress auto-ignition will dependupon the particular application and, in particular, upon factors such asthe quality of the fuel, the mixing/vaporization scheme, the distancethe vaporized gas stream must travel to reach the combustor, the heat ofthe vaporized gas stream as it leaves the vaporizer, the heat to whichthe reduced oxygen gas stream/fuel mixture is subjected in the combustorprior to combustion, and the distance from the pre-mixing zone to thecombustion zone in the combustor.

As discussed above, the combustor 5 of FIG. 1( a) may be a premixedcombustor as shown in FIG. 1( b). Premixed combustors typically containa premixing zone 5 b-1, a primary combustion zone 5 b-2, an intermediatezone 5 b-3 and a dilution zone 5 b-4. In a premixed combustor, thereduced oxygen vaporized fuel gas stream 8 is fed to the premixing zone5 b-1, where it is premixed with an oxygenated gas stream 9 a (e.g.,air). The oxygenated gas stream 9 a is typically fed to some or all ofthe other zones 5 b-2,5 b-3,5 b-4. In an RQL combustion device, thereduced oxygen vaporized fuel gas stream 8 is also fed to theintermediate zone 5 b-3. Alternatively, the combustor 5 of FIG. 1( a)may be a diffusion combustor, as shown in FIG. 1( c), including aprimary combustion zone 5 c-1, an intermediate zone 5 c-2, and adilution zone 5 c-3. In a typical diffusion combustor, the reducedoxygen vaporized fuel gas stream 8 is fed to the primary combustion zone5 c-1, where it is combusted in the presence of the oxygenated gasstream 9 a.

FIG. 2 shows a flow chart of a method of operation of a liquidfuel/reduced oxygen gas vaporization system, in accordance with oneexemplary embodiment of the present invention. A reduced oxygen gasstream and a feed from a liquid source fuel source are each supplied tothe liquid fuel vaporization unit at step 10. The liquid fuelvaporization unit mixes and vaporizes the supply streams at step 11. Thevaporization energy may be supplied by the reduced oxygen gas stream orfrom another energy source. The vaporized fuel stream, which has beenconditioned to avoid auto-ignition by mixing with the oxygen-reducedstream, is then fed to a combustor at step 12. The combustor uses theprepared liquid fuel/reduced oxygen gas stream with an oxygen source tocreate a combustible mixture at step 13.

Another embodiment of a combustion system according to the presentinvention is shown in FIG. 3. The combustion system of FIG. 3 includes aconventional gas turbine engine 14 having an air compressor 15(connected to a combustion air supply, not shown in FIG. 3), a combustor5 (which, as discussed above, may be a premixed or diffusion combustor),a turbine 16, and a stack 17 for emission release. The turbine engine 14can be coupled to any device, e.g., to a generator 18 or other output,such as a naval vessel's screws. In this embodiment, a portion of theexhaust stream 20 from the stack 17 is used to supply the reduced oxygengas stream to a liquid fuel/liquefied gas vaporization unit 21. Theliquid fuel/liquefied gas vaporization unit 21 is connected to theconventional gas turbine engine 14. The vaporization unit 21 includes acompressor 19 to pressurize the stack exhaust stream 20, a fuelvaporizer 4, and a liquid fuel/liquefied gas source 3, which may becontained within the unit 21 or, alternatively, separate from andconnected to the unit 21.

FIG. 4 shows a flow chart of one method of operation of a liquidfuel/reduced oxygen gas vaporization system for use with a turbine, inaccordance with an embodiment of the present invention. The turbineexhaust stream, which has reduced-oxygen content, is fed to a compressorat step 25. The compressor pressurizes the gas turbine exhaust stream atstep 26. The compressor output of the resulting oxygen-reduced streamand the liquid fuel stream are each fed to the liquid fuel vaporizer atstep 27. The compressor output is mixed with the liquid fuel stream tovaporize the liquid fuel at step 28. The reduced oxygen vaporized liquidfuel stream is then fed to the combustor of the gas turbine at step 29.

In some preferred embodiments, the turbine engine 14 is an existinglean, premixed device configured to operate with natural gas, and theliquid fuel 3 is a higher hydrocarbon liquid fuel. In addition to theaforementioned auto-ignition problem, a second issue arises inconnection with the use of higher hydrocarbon fuels in combustiondevices configured to operate with natural gas—because higherhydrocarbon fuels have a higher energy content than natural gas, thefuel gas distribution and metering system of an engine configured tooperate with natural gas would normally require modification to operatewith a higher hydrocarbon fuel gas. However, in preferred embodiments,the gas vaporization unit 21 is configured to supply a reduced oxygenvaporized fuel gas to the turbine engine 14 such that no modification tothe fuel gas distribution system of the engine 14 is necessary. This isaccomplished by mixing an amount of reduced oxygen gas with thevaporized fuel such that the energy content of the reduced oxygenvaporized fuel gas from the vaporizer 4 is equivalent to natural gas.This may be done on a volumetric or mass basis, depending upon the fuelmetering method used by the engine 14. In other embodiments, the energycontent of the reduced oxygen fuel gas is higher or lower than that ofnatural gas and the fuel distribution system is configured to operatewith such higher or lower energy content gas.

By way of example, the heating value of a fuel gas is approximatelyproportional to the number of carbon atoms in the gas molecule.Therefore, pentane (C₅H₁₂) has approximately 5 times the heating valueof the primary component of natural gas, methane (CH₄). If liquefiedpentane were used as the liquid fuel in the system of FIG. 3, thevaporizer 4 would be configured to output a fuel gas stream comprisingone part vaporized pentane gas and four parts reduced oxygen gas for usewith an engine 14 having a fuel gas distribution system configured formetering methane on a volumetric basis.

FIG. 5 a illustrates yet another embodiment a combustion systemaccording to the present invention including a gas turbine engine 14having a compressor 15, a combustor 5, a turbine 16, and a stack 17 foremission release. The turbine 16 can be coupled, for example, to agenerator 18 or any other device, such as a naval vessel's screws. Aliquid fuel/liquefied gas vaporization unit 31 of one embodiment of thepresent invention is connectable to the gas turbine engine 14. In theembodiment shown in FIG. 5 a, the unit 31 includes an air separator 32,an auxiliary compressor 33, a second compressor 34, a fuel vaporizer 4,and a liquid fuel/liquefied gas source 3, which may be contained withinthe unit 31 or, alternatively, separate from and connected to the unit31.

The air separator 32 intakes a compressed air stream from the compressor15 of the engine 14 (or a compressed air stream from another source),and outputs an oxygen rich gas stream 41 and a reduced oxygen gas stream42, which typically contains a high amount of nitrogen relative to air.A wide variety of air separators are known in the art. In someembodiments, the air separation unit produces the oxygen-rich andreduced oxygen streams 41, 42 using a process referred to as adsorption.In such embodiments, the air stream may be compressed to a pressure ofthree atmospheres to facilitate separation.

In the embodiment of FIG. 5 a, the oxygen-rich stream 41 is compressedand the compressed oxygen-rich gas stream 43 is injected into thecombustor 5. The oxygen-reduced stream 42 is fed to the auxiliarycompressor 33, where it is pressurized. The resulting compressedoxygen-reduced gas stream 45 is then fed to the liquid fuel/liquefiedgas vaporization unit 4. The liquid fuel/liquefied gas vaporization unit4 mixes liquid fuel/liquefied gas feed 6 from a liquid fuel/liquefiedgas source 3 with the compressed oxygen-reduced stream 45 at an elevatedtemperature to evaporate the liquid fuel/liquefied gas. The ratio atwhich the compressed oxygen-reduced stream 45 and gas feed 6 are mixedis dependent upon the liquid fuel 3 and the configuration of the engine14. As discussed above, the ratio may be selected to allow an engine 14that is configured to combust natural gas to be used with a higherhydrocarbon liquid fuel 3 without modification to the fuel distributionsystem of engine 14. The vaporization fuel/oxygen-reduced stream 8 isthen fed to the combustor 5.

FIG. 6 shows a flow chart of a method of operation of a liquidfuel/liquefied gas vaporization system for use with a turbine, inaccordance with an embodiment of the present invention. As shown in FIG.6, compressed air is taken from the air compressor of the gas turbineengine at an appropriate stage/pressure for use in the air separationunit at step 51. The air separation unit takes the compressed air streamand creates an oxygen-rich stream and an oxygen-reduced stream at step52. In one embodiment, the oxygen-rich stream is fed to a firstauxiliary compressor at step 53, the first auxiliary compressorpressurizes the oxygen-rich stream at step 54, and the pressurizedoxygen-rich stream is then injected into the combustor at step 55. Insome embodiments, the oxygen rich fuel stream is injected into thecombustor 5 downstream of the flame front (e.g., an intermediate zone ordilution zone of a combustor, such as a premixed combustor as shown inFIGS. 5( b) and 5(c), respectively, or a diffusion combustor) to reducethe amount of pollutants emitted by the engine 14. In other embodiments,the oxygen rich fuel stream is mixed with the combustion air fromcompressor 15 that is fed to the primary combustion zone of thecombustor 5 as shown in FIG. 5( d) (premixed combustor) and FIG. 5( e)(diffusion combustor). This widens the operating range of the combustor,which allows combustion to occur at a lower equivalence ratio (i.e.,leaner combustion), which can lower the emission of pollutants such asNO_(x). In yet other embodiments, the oxygen rich fuel stream is simplymixed with the air from the compressor 15 and fed to all zones of thecombustor.

The oxygen-reduced stream from the air separation unit is fed to asecond auxiliary compressor at step 56, and the second auxiliarycompressor pressurizes the oxygen-reduced stream at step 57. Theresulting compressed oxygen-reduced stream, along with a liquidfuel/liquidized gas stream from a liquid fuel source, are then fed tothe liquid fuel vaporization unit at step 58. The liquid fuelvaporization unit mixes the fed liquid fuel/liquidized gas stream withthe compressed oxygen-reduced stream at an elevated temperature toevaporate the liquid fuel/liquidized gas at step 59. In an embodiment ofthe present invention, the degree to which the oxygen-reduced stream andthe liquid fuel/liquidized gas are mixed is adjustable to specificheating value and/or mass or volumetric flow rate specifications asappropriate for various liquid fuels/liquefied gases. The vaporizedfuel/oxygen-reduced stream is then fed to the combustor through, forexample, the existing natural gas fuel system for use in the turbine atstep 60.

FIG. 7 illustrates an embodiment of the invention in which a fuelvaporizer 4 is supplied with a reduced oxygen gas stream from apre-burner 701. The pre-burner 701 is supplied with air for combustionby the gas stream 9.

As discussed above, some embodiments of the invention are configured toproduce oxygen-reduced fuel gas streams from liquid fuels that can befed to existing combustion devices such as gas turbine engines that areconfigured to combust other fuels such as natural gas withoutmodification to the existing combustion devices. This is accomplished bymixing the fuel gas with an inert, reduced oxygen stream to keep theenergy content of the fuel gas equal to that of natural gas on either amass or volumetric basis depending upon the metering method used by thecombustion device. For example, FIG. 8 illustrates another embodiment ofthe invention in which a combustor 5 is fed by a fuel metering system801 that is configured for natural gas. In most existing combustiondevices, fuel gas/combustion air ratio can be controlled such that themixture may be made more or less lean. An additional benefit of thepresent invention is that many of the reduced oxygen vaporized higherhydrocarbon fuels can be burned at an equivalence ratio lower (leaner)than that of methane under the equivalent conditions (i.e., sametemperature, same combustion air (or other oxygen containing gas)supply, etc.). For example, at atmospheric pressure the minimumequivalence ratio of methane is typically about 0.5 in air, while manyhigher hydrocarbons fuels can be combusted at an equivalence ratio ofapproximately 0.45 in air. The use of lower equivalence ratios reducesthe emission of pollutants such as NO_(x). As discussed above, theoperating equivalence ratio of the combustion device may be even lowerin embodiments in which the operating range has been widened through theaddition of an oxygen rich stream from an air separator to thecombustion air stream.

In other embodiments of the invention, a reduced oxygen fuel gas with ahigher or lower energy content than that of natural gas is produced. Insuch embodiments, if a combustion device configured to run on naturalgas is used, the fuel distribution/metering system of the combustiondevice may need to be appropriately modified.

Example embodiments of the present invention have now been described inaccordance with the above advantages. It will be appreciated that theseexamples are merely illustrative of the invention. Many variations andmodifications will be apparent to those skilled in the art.

1. A method for operating a combustion device, the method comprising thesteps of: producing a fuel gas using a liquid fuel comprisinghydrocarbon molecules and a diluent gas; premixing the fuel gas with asecond gas containing oxygen to produce a gas mixture in a premixingzone located upstream of a combustion zone of a combustion device, thepremixing zone being configured such that autoignition of the gasmixture would occur in the absence of the diluent gas; and combustingthe gas mixture in the combustion zone of the combustion device; whereinthe diluent gas is inert and present in an amount such that reaction ofthe fuel gas upstream of the combustion zone is substantiallysuppressed.
 2. The method of claim 1, wherein the gas mixture has anamount of oxygen sufficient to support combustion of the gas mixture. 3.The method of claim 1, wherein the gas mixture is a lean mixture with anequivalence ratio less than
 1. 4. The method of claim 1, wherein atleast 50 weight percent of the hydrocarbon molecules of the liquid fuelhave at least two carbon atoms.
 5. The method of claim 1, wherein theoxygen content of the gas mixture is below a limiting oxygen index ofthe liquid fuel.
 6. The method of claim 1, wherein the combustion deviceincludes a fuel metering apparatus configured for natural gas.
 7. Themethod of claim 1, wherein an equivalence ratio of the gas mixture isless than a minimum equivalence ratio at which methane could becombusted under equivalent operating conditions.
 8. The method of claim1, wherein the combustion device is a gas turbine engine.
 9. Theapparatus of claim 1, wherein the liquid fuel is a liquified gas, theliquified gas being of a composition that would be in a gaseous state atroom temperature under atmospheric pressure.
 10. A combustion apparatuscomprising: a combustor, the combustor having a first inlet foraccepting fuel gas, a second inlet for accepting oxygenated gas forsupporting combustion of the fuel gas, a combustion zone, and apremixing zone upstream of the combustion zone, the combustion devicebeing configured to premix the fuel gas with at least some of theoxygenated gas in the premixing zone to produce a gas mixture, and tocombust the gas mixture in the combustion zone; and a fuel vaporizationunit in fluid communication with the inlet of the combustor, the fuelvaporization unit being configured to produce fuel gas using a liquidfuel comprising hydrocarbon molecules and a diluent gas; wherein thepremixing zone is configured such that autoignition of the gas mixturewould occur in the absence of the diluent gas and wherein the diluentgas is inert and present in the fuel gas in an amount such that reactionof the fuel gas upstream of the combustion zone is substantiallysuppressed.
 11. The combustion apparatus of claim 10, further comprisinga fuel gas metering apparatus for controlling the supply of fuel gas tothe first inlet, the fuel gas metering apparatus being configured fornatural gas.
 12. The apparatus of claim 10, wherein the combustiondevice is configured to combust the gas mixture at an equivalence ratioless than
 1. 13. The apparatus of claim 10, wherein at least 50 weightpercent of the hydrocarbon molecules of the liquid fuel have at leasttwo carbon atoms.
 14. The apparatus of claim 10, wherein an oxygencontent of the fuel gas is below a limiting oxygen index of the liquidfuel.
 15. The apparatus of claim 10, wherein an equivalence ratio of thegas mixture is less than a minimum equivalence ratio at which methanecould be combusted under equivalent operating conditions.
 16. Theapparatus of claim 10, wherein the liquid fuel is a liquified gas, theliquified gas being of a composition that would be in a gaseous state atroom temperature under atmospheric pressure.
 17. The apparatus of claim10, wherein the combustion device is a gas turbine engine.
 18. Theapparatus of claim 10, wherein the combustion device is configured suchthat an amount of the oxygenated gas sufficient to support combustion ispremixed with the fuel gas in the premixing zone.